1. Field of the Invention
The present invention relates to a method and a system for forming a copper indium gallium sulfur selenide absorption layer and a cadmium sulfide buffer layer, and more specifically to a method and a system which carry out a two-step selenization/sulfurization reaction under non-vacuum condition to form an absorption layer of good quality.
2. The Prior Arts
Due to a rising global trend in green energy, the copper indium gallium diselenide (CIGS) solar cell, which is a complex quaternary alloy, has been studied and developed as a potential energy source in industry. The CIGS solar cell is not restricted by the raw materials, and can be manufactured on a large-area flexible substrate. The conversion efficiency per unit solar cell is up to 20%, and the conversion efficiency of a solar cell module is also up to 14%.
Referring to FIG. 1, a conventional CIGS solar cell is illustrated. As shown in FIG. 1, a CIGS solar cell includes a substrate 10, a back electrode layer 20, an absorption layer 30, a buffer layer 40, a transparent conductive layer 50, and a front electrode layer 60 sequentially stacked from bottom to top. The incident light L enters the CIGS solar cell from the top.
The substrate 10 is generally made of glass. Molybdenum is used for making the back electrode layer 20. The absorption layer 30 consists of a quaternary compound including the elements of copper, indium, gallium and selenium, and the absorption layer 30 is an N type semiconductor layer. The buffer layer 40 includes zinc sulfide, which is a P type semiconductor layer. The transparent conductive layer 50 includes indium tin oxide (ITO) or zinc oxide (ZnO). The front electrode layer 60 may be made of nickel and aluminum, and it is usually formed as a mesh-like pattern. First, a nickel layer is formed on the transparent conductive layer 50 to avoid forming metal oxides having high resistance, and then an aluminum layer is formed thereon.
There is a p-n junction 35 between the N type absorption layer 30 and the P type buffer layer 40 for absorbing the incident light to generate the free electron-hole pairs. Moreover, the back electrode layer 20 has a positive voltage and the front electrode layer 60 has a negative voltage. The back electrode layer 20 and the front electrode layer 60 connect to an outside load through the positive contact 22 and the negative contact 62, respectively, for outputting electric power.
Conventionally, the processes for fabricating CIGS solar cell can be generally divided into the vacuum processes and the non-vacuum processes. In the vacuum processes, the evaporation method and sputtering method are mainly used. However, in the vacuum processes, the expensive process equipments are required, the material utilization efficiency is low, and the fabricating cost is high. In the non-vacuum processes, the ink method is generally used and by which the manufacturing cost can be reduced, and thereby the non-vacuum processes have great development potential. However, the compactness of the formed CIGS absorption layer is relatively low so that the quaternary compound with the large particle size is not easily formed. As a result, many grain boundaries exist in the absorption layer, and when the light falls onto these grain boundaries, it may not be successfully converted into electric energy so that the photovoltaic conversion efficiency becomes low.
Therefore, it is needed a method and a system which may improve the absorption efficiency and the photoelectric conversion efficiency of the CIGS absorption layer under non-vacuum condition, and provide a good lattice match of cadmium sulfide (CdS) buffer layer with CIGS absorption layer to form a better p-n junction, so as to further solve the above problem of the prior art.